greater than 7kV and 11kV, there are two inflection points in
the decrease progress of DC component, which are about
25kV and 15kV. This can indirectly indicate that the DC
component plays a major role in the breakdown of the
insulation when the AC component is between 0~7kV and AC
component have larger effect on the insulation breakdown
when the AC component is between 11~14kV.